Plasma accelerator for wind tunnel



June 23, 1964 Q FONDA-BONARD; 3,138,019

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United States Patent Oiice ,Y 3,138,019 Patented June 23, 1964 3,138,019 PLASMA ACCELERATOR FDR WIND TUNNEL Giusto Fonda-Bonardi, Los Angeles, Calif., assignor to Litton Systems, Inc., Beverly Hills, Calif. Filed Nov. 7, 1960, Ser. No. 67,626 11 Claims. (Cl. 73-147) This invention relates to plasma accelerators, and more particularly to accelerators having a large capacity.

For certain applications such as supersonic wind tunnels, it is necessary to accelerate relatively large masses of air to velocities well above the velocity of sound in air. Plasma accelerators have been proposed heretofore which are capable of accelerating gas to the required speeds. In this regard, reference is made to Siegfried Hansen patent application, Serial No. 723,018, tiled March 21, 1958, entitled Plasma Accelerators, now Patent No. 2,992,345, granted July 1l, 1961, and assigned ,to the assignee of the present patent application. The

accelerator of this prior application included an elongated electromagnetic structure of constant cross-section or of convergent geometry, and arrangements for providing an accelerating field along the length of the magnetic structure to accelerate the plasma. While this structure was well adapted for accelerating pulses of highly ionized plasma to very high velocities, approaching the velocity of light, its capacity for the economical acceleration of large masses of air is severely limited.

Accordingly, a principal object of the present invention is to increase the capacity of plasma type gas accelerators.

Before proceeding with a description of the present invention, it is considered desirable to dene the word plasma which will be frequently used in the present specification; As herein utilized, the term plasma designated a volume of gas in which an appreciable percentage of the atoms are ionized, but which contains the detached electrons within the same volumeV so that the gas as a Except lfor mechanical rigidity, therefore, a plasma is very similar to a metal in its properties since the detached electrons remain uncombined for relatively long periods and can move about at random within the boundaries of the gas volume. In fact, it can readily be demonstrated that the electrical conductivity of a relatively highly ionized plasma `can actually exceed that of such good electrical conduc- .tors as copper and silver.

In accordance with one illustrative embodiment of the present invention, the object set forth above is achieved by providing an elongated electromagnetic structure of vprogressively increasing diameter, to which ionizable gas is supplied at succesive points along its length. Thus, for example, the coil structure may include a rst short accelerator of small cross-section, followed by another accelerator of larger cross-section. Gas accelerated by the iirst accelerator is fed through the center of the second accelerator together with an additional outer sleeve With increas- Accordingly, the second accelerator operates principallyon the peripheral sleeve of gas and brings it up to the velocity of the inner core which was accelerated by the liirst accelerator.

When it is desired to have larger cross-sectional areas of accelerated gas, three or more successive accelerators -may be employed in tandem, with gas being introduced yperipherally at the beginning of each short accelerator.

Alternatively, a single long accelerator of generally conical or funnel-shaped cross-section may be provided, with arrangements for introducing gas along its length. From one viewpoint, the funnel-shaped accelerator may be considered tobe the limiting case of the tandem electromagnetic structure, as the number of coils is progressively increased and the length of each coil is correspondingly reduced.

In accordance with an important feature of the invention, therefore, a high capacity plasma accelerator includes an elongated electromagnetic structure having a small input cross-section and an increasing cross-section toward its output, and also includes arrangements for introducing ionizable material into said structure at its input and at least one additional point, or region, along its length.

In accordance with collateral features of the invention, the electromagnetic accelerator structure of the preceding paragraph may include a plurality of coils of progressively increasing diameter, or it may be a single structure of progressively increasing diameter.

Other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description and from the drawings in which:

FIG. l is a plot representing the turns distribution of a poly-phase field winding of a linear plasma accelerator;

FIG. 2 represents the plasma density, the pressure, and the current density of a high velocity or hard plasma accelerator plotted against distance along the length of the accelerator;

FIG. 3 shows a similar set of plots for a slower speed, or soft plasma accelerator;

FIG. 4 represents a plasma accelerator, in accordance with the present invention, in which gas is introduced at several zones along the length of the accelerator structure; and

FIG. 5 is another embodiment of the present invention in which a generally conical accelerating structure is employed.

Referring more particularly to the drawings FIG. 1 represents the distribution of turns along the length of a hollow cylindrical plasma accelerator. With a polyphase energizing signal, an accelerating eld is produced within the electromagnetic structure. When ionized gas is introduced into the hollow coil structure at one end, it is accelerated down thelength of the coil and ejected from the other end at high velocity. The operation of such an accelerator is disclosed in detail in the co-pending application of Siegfried Hansen cited above. l

Depending on the rate of acceleration and the pressurewithin the accelerator, two distinct modes of operation are possible: in `the case of very high accelerating forces, the gas plasma will be broken into discrete plasma clouds or pulses which are separated from one another by regions which are essentially evacuated. Such Van accelerator is known asa hard accelerator. If lower accelerating forces are employed, however, the ionized zones of plasma merely act as pistons Vand carry additional gas which may not be ionized between the concentrated clouds of ionized gas. rSuch a relatively slow speed plasma accelerator is known as a sof accelerator.

The plots of FIGURES 2 and `3 represen-1: operating conditions for hard and soft accelerators, respectively. The upper and lower curves of FIGURES 2 and 3 are not significantly different, with the exception that the current density and plasma density peaks tend to be more sharp and less diffuse in the case of the hard accelerator of FIG. 2 than in the case of the soft accelerator of FIG. 3. With regard to the central pressure curve in FIGURES 2 and 3, however, the distinct pressure peaks in the hard accelerator are clearly visible in FIG. 2, whereas the pressure gradient between successive pulses produces the generally sawtoothed characteristic shown in the central plot of FIG. 3. The physical reason for the discontinuous pressure peak-s in the central characteristic of FIG. 2, for the hard accelerator, is in part a result of the physical impossibility of having negative pressures, when the accelerating force exceeds the available pressure force.

By way of example, the following two sets of data for hard |and soft accelerators, respectively, have been developed. They will be set forth in the following Tables I and II:

Average mechanical force l Newtons. Average accelerating magnetic field .05 Weber/ m.2.

Average ring current in plasma-. 200 Amp. Plasma temperature after pinch 1390 K. IPlasma pressure after pinch 2.91 Newton/m2. Pinch radius 1.48X *2 m. Pulse power demand 500 k-w. Average outputpower 600 watts. 4Rate of'heat generation in ring `17.35 kw.

Table Il Output density 4.10X10d4 kg./meter3. Output pressure 3.73 X 10-4 atm. Output temperature 349 degrees K. Output velocity Mach =20 375 y:7500 meter/ sec. `Output mass 3.07 X 10'2 kg./sec. Output volume 75 meter3/sec. Output area 1X 10-2 meter2. Accelerating power 1020 kw. Heating power (122 K.-349 K 8 5 kw, Ion density in rings 99.1%, Average ion density 17% Exciting frequency 10 kilocycles. Mass of each gas ring 1 53 X10-6 kg -Disassociated charge in each ring 1.7 Coulomb. Ring Current 32.8 amperes. Electrn drift Speed 10,5 meter/sec at 0.175 meter diam. Ring voltage.V 3.72 `10-4 volt. Power dissipated in each ring 1 22X10-2 watt, lPeak UX density 0.07 Weber/ meter2. Average uX at fing 0.05 Weber/ meterz. 'Core losses 1,7 watt/1b, Average acceleration 1 '07X106 meter/Seg Duration for 7500 meter/ sec.

Power dissipated in rings 1,43 watts,

Expansion nozzle (input to accelerator):

Output area 4.24X10*2 meter2.

lengths.

4 Table II-Continued Associated equipment:

With regard to the foregoing two sets of data, they are merely illustrative of particular sets of operating conditions for single stage plasma accelerators. The relatively high cooling requirements are particularly to be noted. In the case of wind tunnel operation, testsfor relatively short periods of vtime areIoften adequate. If prolonged operation is undertaken, however, adequate cooling must of course be provided.

The present invention involves an improvement over the plasma accelerators of the prior art, in which large masses of air may be accelerated by means of a plasma accelerator. FIG. 4 represents one illustrative embodiment of my invention. In the apparatus of FIG. 4, it is desired to produce a large volume of air at the output 12 from the accelerator which has a speed of about 7500meters per second. This would correspond to approximately Mach 20 under pressure conditions corresponding to an elevation of approximately 200,000 feet.

In the operation of plasma accelerators, the accelerating electromagnetic structure tends to be operative only within a relatively short distance of the electromagnetic turns. Thus, in the case of very large diameter coils, the central core of air is not accelerated to the desired speed and an undesirable variation in the velocity of gas is observed across the output channel from the accelerator. In the arrangement of FIG. 4, three successive accelerator structures 14, 16 and 18, are provided which accelerate the central core of air and then successive additional sleeves of air which are introduced between the successive coil structures.

Thus, for example, the central core of air accelerated by the coil structure `14 is provided from the annular nozzle structure 20. Coupled to the nozzle structure 20 is an annular ionization chamber V22. Ionization cham- 'ber 22 includes an electrode 24 which is energized through a series of conductors 26, 28 to a potential at which discharge will occur at the relatively low pressure within the chamber 22. Ionizable gas, such as air, is supplied to the ionization chamber 22 through inlet tubes 30, 32 from the manifold structure 34.

A similar gas inlet structure 36 is provided for supplying gas to the space between coils 14 and 16; similarly, the structure 38 supplies partially ionized gas to the annular space between electromagnetic structures 16 and 18. With the exception of increased size, the structures 36 and 38 are similar to the structure including the nozzle 20 and ionization chamber 22 described above. Each of the three structures makes a gas tight tit with its associated coil structure. Air is fed from the input line 40 to the manifolds 34, 42 and 44 associated with the successive gas inlet arrangements. Through the use of a common input conduit 40, accurate control of pressure to the three structures may be provided. As indicated by the dotted lines 46 within the coil structure 14, the annular input gas stream from the nozzle structure 20 merges to form a central cylindrical core of gas. Similarly, as indicated by the dotted lines 48, the gas from the ionization structure 36 merges with the gas accelerated by the upper electromagnetic structure 14. Thus, at the output 12 from the electromagnetic 18, the gas is owing in a uniform parallel path into the wind tunnel structure 52.

The coil structures 14, 16 and 18 may each be pro- `vided with three electromagnetic windings along their The distribution of these windings may be as indicated in FIG. 1 and as described in more detail in the co-pending patent application of Siegfried Hansen citedabove. The windings in the electromagnetic structures 14, 16 and 18 may be energized from three phase .electrical signal generator 54.

As indicated in the foregoing description, a large volume of uniformly accelerated air is supplied to the wind tunnel structure 52 by the three coil structures 14, 16 and 18 The amount of air supplied to the wind tunnel is economically accelerated by the indicated structure, principally as a result of the successive gas inputs between the tandem electromagnetic coils. Thus, as briefly mentioned above, an electromagnetic coil tends to act on an ionized plasma only in the region moderately close to the coil. Thus, if a single large coil such as coil 18| were employed, the air toward the center of the coil would not be accelerated to'the proper high velocity. Particularly in view of the location of models to be tested in the center of the wind tunnel structure 52, it is irnportant to have uniform high speed gas across the outlet 12 from the electromagnetic structure. Arrangement of the present invention produces this desired type of output gas ow.

For wind tunnel applications, gas distribution along the acceleration structure is preferably of the form indicated by the plots of FIG. 3. Thus, the pulses of plasma passing down the electromagnetic acceleration structure serve as pistons for entrapped air. This mode of operation is indicated by the central plot of pressure in FIG. 3. Following a short drift space within the Wind tunnel structure 52, gas diiusion tends to level out even the shallow pressure and density fluctuations, both across the wind tunnel, and longitudinally with respect to gas flowing from the accelerator, as indicated in FIG. 3.

As mentioned above, the series of three tandem coils shown in FIG. 4 may be increased in number and each of the coils shortened. With these changes in construction, the variations in pressure are signicantly reduced. A conical coil structure may be considered to be in the limiting case of such a change, and is to be preferred, particularly from the standpoint of reduced pressure and density variations.

An embodiment of the invention which includes a ared or conical coil, is shown in FIG. 5. In the arrangement of FIG. 5, a generally conical coil structure 60 is provided in place of the series of coil structures of progressively increasing size, shown in the arrangement of FIG. 4. The structure of FIG. 5 is similar to that of FIG. 4 in that gas is introduced progressively along the length of the acceleration structure. This is accomplished in the structure of FIG. 5 by a gas distribtl tion system including a plenum, or pressure chamber 62 and a porous or perforate shield 64. Ionizable gas such as air is supplied to the plenum chamber 62 and 1s then directed both to the apeX 66 of the coil structure 60 and through the openings in the shield 64 to pass through the coil structure along its length.

The plenum chamber 62 is made of insulating material to permit ionization of the incoming gas by the encircling electromagnetic coil 68. The coil 68 is powered by a suitable high frequency power source 69. Electrostatic pre-ionization structures such as those shown in FIG. 4 could also be used. If they are employed, the plenum chamber would be of metal. Alternatively, the preionization structures may be dispensed with and the high frequency signals supplied to the coil 60 could be relied upon for ionization. As disclosed yin D. Curtis et al. application Serial No. 67,645, entitled Ionization and Plasma Acceleration Apparatus, and led concurrently with this specification, the power source 70 may be a single phase power supply and may serve to both ionize the gas and to accelerate the resultant plasma. As eX- plained in detail in the Curtis case, the Haring nature of the coil provides an outward component of velocity which accelerates the gas plasma from the mouth 72 of the funnel-shaped structure.

In addition to the components specifically discussed 6 above, the acceleration structure of FIG. 5 includes supporting members 74 and 76 for the shield 64 and coil 60. The supporting structure 74 may be a series of rods, or the like, to permit the free passage of air to the openings in the shield 64. An outer housing 78 is also provided to prevent the escape of gas.

The wind tunnel structure 80 is directly connected to the output 72 of the accelerator. As mentioned above, a reasonable drift space is provided between the output 72 of the accelerator and the aerodynamic model 82 under test. This permits a reduction in the variations ,in gas pressure which may be present as the gas iiows from the accelerator. Conventional supporting arrangements 84 and sensing and recording apparatus 86 are provided for mounting the model 82 and for recording its aerodynamic properties. Conventional vacuum pumping apparatus 88 of high capacity is also provided for producing the necessary reduced pressures within the wind tunnel 80.

It is to be understood that the above-described arrangements are illustrative of the application of the principles of the invention. Numerous other arrangements may be readily devised by those skilled in the art without departing from the spirit and scope of the invention.

Thus, by way of example, but not of limitation, additional electro-magnetic acceleration structures may be provided in the arrangement of FIG. 4 described above and three-phase windings could be employed in the apparatus of FIG. 5. Accordingly, it is to be understood that the present invention is limited only by the spirit and scope of the appended claims.

What is claimed as new is:

l. In a high capacity plasma accelerator, an elongated hollow electromagnetic acceleration structure having an input end of small cross-section and an output end of significantly greater cross-section, means for introducing ionizable material into said structure at the input end of said structure, and means for introducing additional ionizable material into said structure toward the output end thereof.

2. A combination as defined in claim 1 wherein said structure comprises a progressively flared coil, and wherein means are provided for introducing gas along the length of said coil.

3. A combination as defined in claim l wherein said structure includes a plurality of discrete electrical coils of progressively increasing diameter, and wherein gas is introduced at the beginning of said smallest coil and between successive coils.

4. In a high capacity plasma accelerator, an elongated electromagnetic acceleration structure having a narrow input cross-section and an increasing cross-section toward the output of said structure, and means for introducing ionizable material into said structure at the reduced input to said structure.

5. In a high capacity plasma accelerator, an elongated hollow electromagnetic acceleration coil structure having an increasing cross-section from its input to its output, means for introducing ionizable material into said structure at the reduced input to said structure, means `for supplying high frequency electrical signals to said coil structure, and means for providing subatmospheric pressure conditions within sa-id coil structure.

6. In combination, a wind tunnel, means for mounting an aerodynamic model to be tested in said wind tunnel, Vacuum pumping equipment coupled to said tunnel to reduce the pressure in said tunnel, and a plasma acceleration coil structure having a small input cross-section and an output cross-section of significantly greater area, the output of said coil structure being coupled to said wind tunnel.

7. A high capacity plasma accelerator comprising an elongated hollow electromagnetic acceleration coil structure having an increasing cross-section from its input end to its output end, means for introducing ionizable mate- 7 rial into said structure at the reduced input end of said structure, `means for supplying high frequency electrical signals to said-coil structure, means for pre-ionizing said ionizable material prior to introduction into said structure, and means for providing subatmospheric pressure conditions within said coil structure.

8. In combination, `a Wind tunnel, means for mounting an aerodynamic model to be tested in said `wind tunnel, vacuum pumping equipment coupled to said tunnel to reduce the pressure in "said tunnel, a plasma acceleration coil structure having a small input cross-sectional area and an output end having a cross-section of significantly greater area, the outputof said coil structure being coupled to said wind tunnel, and means for supplying ionizable material to said coil structure at its small input end `and at another point along the length of said coil structure.

9. A combination `as dened in claim 8 wherein said coil structure is a progressively ared coil.

10. A combination as defined `in claim 8 wherein said 'means for supplying ionzable material constitutes an air distribution system.

11. in combination, a channel, vacuum pumping equipment coupled to said channel to reduce its-pressure, a

plurality of discrete plasma acceleration coil structures,

lto said channel.

References Cited in the file of this patent UNITED STATES PATENTS Edstrom Jan. 10, 1961 Hansen July 11, 1961 

8. IN COMBINATION, A WIND TUNNEL, MEANS FOR MOUNTING AN AERODYNAMIC MODEL TO BE TESTED IN SAID WIND TUNNEL, VACUUM PUMPING EQUIPMENT COUPLED TO SAID TUNNEL TO REDUCE THE PRESSURE IN SAID TUNNEL, A PLASMA ACCELERATION COIL STRUCTURE HAVING A SMALL INPUT CROSS-SECTIONAL AREA AND AN OUTPUT END HAVING A CROSS-SECTION OF SIGNIFICANTLY GREATER AREA, THE OUTPUT OF SAID COIL STRUCTURE BEING COUPLED TO SAID WIND TUNNEL, AND MEANS FOR SUPPLYING IONIZABLE MATERIAL TO SAID COIL STRUCTURE AT ITS SMALL INPUT END AND AT ANOTHER POINT ALONG THE LENGTH OF SAID COIL STRUCTURE. 